A signal comprising a single pulse having the waveform shown in FIG. 1(a) may be represented in the frequency domain by the spectrum shown in FIG. 1(b). The frequency .omega..sub.o at which the amplitude of the spectrum starts to roll off is determined by the rise time and fall time of the pulse. A signal which comprises a periodic succession of pulses each having the waveform shown in FIG. 1(a), for example as shown in FIG. 1(c), has the form of a comb when represented in the frequency domain (FIG. 1(d)). The shape of the envelope of the comb is determined by the single pulse's spectrum. The spacing between the lines of the FIG. 1(d) spectrum is equal to the reciprocal of the interval between successive pulses. Accordingly, a repetitive pulse-form signal having steep transitions may be used as an harmonic generator, and a selected harmonic component may be used as a reference in high resolution time interval measurements. However, interpulse jitter (variation in the interval between successive pulses) causes a broadening of the lines in the spectrum of the repetitive pulse-form signal and reduces the accuracy of time interval measurements carried out using that signal as a reference.
A step-recovery diode can be used to generate a pulse-form electrical signal having steep transitions, e. g. 200 mv/ps. However, the drive circuits that are at present available do not have sufficient short-term stability to take full advantage of the steep transitions that can be generated using a step-recovery diode.
Surface acoustic wave (SAW) resonators have a very high unloaded quality factor (Q). It is not uncommon for a SAW resonator to have a Q as high as 10,000 to 30,000. This makes the SAW resonator-based oscillator useful for generating signals which have good short-term frequency stability, i.e. have low interpulse jitter. However, SAW resonators have a very high insertion loss and insertion delay, and consequently it is difficult to start a conventional oscillator that employs a SAW resonator as its resonant circuit.